Tính
a) \({3 \over 5}.\left( {{5 \over 3} – {2 \over 7}} \right) – \left( {{7 \over 3} – {3 \over 7}} \right).{3 \over 5}\)
b) \({8 \over 9}:\left( {{3 \over 5} – {2 \over 7}} \right) – {8 \over 9}:\left( {{6 \over 7} – {4 \over 5}} \right)\)
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\(\eqalign{ & a){3 \over 5}.\left( {{5 \over 3} – {2 \over 7}} \right) – \left( {{7 \over 3} – {3 \over 7}} \right).{3 \over 5} = {3 \over 5}.\left( {{5 \over 3} – {2 \over 7} – {7 \over 3} + {3 \over 7}} \right) \cr & = {3 \over 5}.\left[ {\left( {{5 \over 3} – {7 \over 3}} \right) + \left( { – {2 \over 7} + {3 \over 7}} \right)} \right] \cr & = {3 \over 5}\left( {{{ – 2} \over 7} + {1 \over 7}} \right) = {3 \over 5}.\left( {{{ – 14} \over {12}} + {3 \over {21}}} \right) = {3 \over 5}.\left( {{{ – 11} \over {21}}} \right) = {{ – 11} \over {35}} \cr} \)